The invention relates to the manufacture of glass and in particular to a method of conditioning molten glass and a glass melting tank for carrying out such conditioning.
In a known method of manufacturing glass in a continuous process, raw materials are fed in at one end of a glass melting tank to form a blanket floating on an existing bath of molten glass. The rate of feeding is sufficient to maintain a constant glass depth in the tank whilst molten glass progressively flows towards the opposite end of the tank known as the working end, from which molten glass is taken away for use in a forming process. The blanket of raw materials is converted to molten glass as it passes through a melting zone at one end of the tank by heat which may come for example from burning fuel supplied from burners situated at spaced intervals in the side walls above the glass level or from electrical heating devices. The molten glass passes from the melting zone into a refining zone where heat is also applied above the molten glass. In the refining zone bubbles of gas still remaining in the glass are encouraged to escape or go into solution in the glass. The glass passes from the refining zone into a conditioning zone adjacent the working end of the tank. In the conditioning zone the glass is homogenised and brought to a suitable thermal condition for use in the forming process. Normally a canal leads from the working end of the tank to a forming process.
From the above, it can be seen that certain regions of the tank are defined as melting, refining and conditioning zones. As regards the molten glass passing from one zone to another, all the glass leaving any one zone may not necessarily have reached a final state for that operation e.g. a fully refined state as it enters the conditioning zone. Some refining can still occur in the conditioning zone, and conditioning may start to some extent in the refining region. Hence the zoned regions are defined to show the areas in which the greater part or all of a particular operation is carried out in a tank, and enables the man practised in the art to identify the temperature conditions required in these zones.
Conventionally flat glass melting tanks are designed to contain a large body of molten glass and are normally of substantially uniform depth throughout the melting, refining and conditioning zones. Convective flow currents set up within the molten glass aid in mixing the glass to achieve homogeneity of temperature and composition. At the same time colder return flows of glass which occur in the lower regions of the tank from the conditioning zone back towards the melting zone protect the refractories at the bottom of the tank from the wear which might otherwise occur if subjected to the higher glass temperatures used in the melting and refining zones.
The above process for the manufacture of glass is however wasteful of heating energy since the returning colder glass in the lower layers of the tank must be reheated each time it passes back through the glass tank. It has been found that the amount of glass which circulates and returns from the conditioning zone to the melting zone is dependent on the depth of the molten glass as well as the temperature gradient between the two ends of the tank and the output from the tank. It is possible to choose the conditions so that all the glass flows in a downstream direction towards the outlet end with no return flow. There are however difficulties in achieving satisfactory homogeneity of temperature and composition if the flow within the tank is entirely in one direction with no return flow. Furthermore, in the conditioning zone it is necessary to lower the temperature of the glass but any excessive surface cooling in the conditioning zone is likely to cause unacceptable inhomogeneity in the molten glass. Furthermore, it is desirable to avoid an excessively long conditioning zone.
Conditioning as a process can vary from achieving substantial thermal and physical homogeneity in glass when it leaves the conditioning zone to achieving a particular temperature gradient in the glass. Conventional methods of achieving a suitably conditioned glass are normally based on supplying cooling air to the surface of the glass as it flows to a forming process. However as the unit output and hence throughout of glass is increased, using conventional air cooling system it has been necessary to increase the size of the conditioning area and provide more accurate control so as to avoid causing steep temperature gradients in the glass due to using large volumes of cooling air on the glass surface. There have been other proposals for removing heat from the bottom of the conditioning zone by use of cooling air and in some cases cooling pipes have been positioned in the glass for the removal of unwanted heat from the glass. However these previous proposals have not involved selective removal of heat from within the body of the glass adjacent the inlet of the conditioning zone to achieve a controlled temperature profile through the glass.
It is an object of the present invention to provide an improved method and apparatus for conditioning molten glass and achieving a desired temperature profile through the glass while all the glass flows in one direction.